This invention relates to the field of optical signal processing, and in particular to the processing of packet-based optical signals and to the reading and updating of the packet headers of such signals.
The advantages of packet-based optical transmission systems are well known, and the SONET standard is widely accepted as a suitable physical layer for optical transmission systems. Indeed, SONET is one of the many physical layers defined for ATM, which is itself a cell-based (i.e. packet-based) switching and multiplexing technology.
The rapid increase in transmission rates achieved by optical transmission systems far exceeds the capability of electronic processing of signals. Consequently, the limitation to data transfer rates results principally from delays introduced by electrical switching elements. However, these electronic and opto-electronic elements are required for performing switching and routing functions, and the conversion of high-speed optical data to electrical signals for the switching and routing operations is recognised as causing a data transfer restriction.
There have been proposals which provide all-optical networks in which switching and routing take place in the optical domain. One proposal involves the use of time-shift keying, but the fine tolerances in timing and delay compensation present serious difficulties.
An alternative proposal described in U.S. Pat. No. 5,541,756 is to provide a packet header with wavelength-coded data, with a grating operating as a wavelength differentiator. Different wavelength signals within the optical header may then be deflected by differing amounts for detection by photo-electric sensors positioned at different locations. A problem with the use of multiple wavelengths within a single optical packet is that the different signals must be spaced by guard bands to take account of the different amounts of optical dispersion of the different wavelength signals.
According to a first aspect of the present invention, there is provided an apparatus for reading data from a packet header of an optical packet for transmission over an optical network, and for writing data to the packet header, data stored in the packet header being encoded using amplitude modulation, the apparatus comprising:
a divider for dividing an incoming optical signal into a first and a second path;
a converter in the second path for converting at least the packet header into an electrical signal;
header reading circuitry;
header updating circuitry;
an erasing unit in the first path for removing the data stored in the packet header by constraining the packet header signal to have a constant amplitude; and
a modulator provided in the first path for modifying the constant amplitude signal using updated header information from the header updating circuitry.
The reading apparatus of the invention only needs to convert the packet header into an electrical signal, in the second path, whereas entire optical packet remains in the optical domain in the first path. The header is thus read in the electrical domain, but it is updated optically. Rewriting of the optical header does not seek to replace the optical carrier, so that there are no issues of carrier superposition.
Preferably, therefore, the carrier wavelength of the packet header and of the payload of the optical packet are the same.
The use of amplitude modulation in the packet header enables amplitude detection using conventional commodity electro-optics.
The amplitude modulation of the data in the header may provide a first maximum amplitude representing a first value and a second amplitude representing a second value, the second amplitude being substantially greater than the minimum value but less than the maximum value. This modulation technique enables data carried by the header to be unwritten without destroying the underlying carrier, for example by reducing the amplitude of the packet header signal to the second amplitude. The use of low modulation depth amplitude modulation in this way gives rise to an additional optical loss introduced by unwriting the optical header, but this represents only a fractional increase in the optical span loss. For example, the first amplitude and the second amplitude may differ by 10% (a 0.5 dB loss) compared to the optical span loss of 23-30 dB. In other words, the unwriting of the optical header still leaves a substantial optical carrier which can be re-modulated with new header information.
Instead of reducing the amplitude of the header data to the second amplitude, the erasing unit may comprise a booster for increasing the amplitude of the packet header signal to the first amplitude, thereby overwriting the packet header information. The modulator may then modulate the booster signal to include updated header information. This arrangement enables the increase in optical loss to be avoided, but of course requires an active optical component.
The booster is preferable gated, so that the boosting operation is controlled to coincide only with the packet header signal, and not with the packet payload signal.
The updated header information may be fed to the second path either in the electrical or the optical domain.
The data may be stored in the payload of the optical packet at a higher rate than in the header. A lower rate header signal enables commodity electro-optics to be used, giving cost advantages through using volume products. The electronics may not respond to the signal in the payload of the optical packet. However, all synchronisation and framing data will be contained within the header signal, so that no information needs to be derived from the optical payload to enable reading of the header information.
The apparatus for reading data may comprise a packet routing apparatus having a plurality of outputs, and which routes an input packet to one or more of the plurality of output depending on the header data read by the header reading circuitry. For example, the apparatus may comprise an optical ATM switch.
The invention also provides a method of reading and updating a packet header of an optical packet for transmission over an optical network, comprising:
dividing an incoming optical signal into a first and a second path;
converting the signal in the second path into an electrical signal and reading the header information;
providing updated header information;
optically removing the data stored in the packet header in the first path by constraining the packet header signal to have a constant amplitude;
modifying the constant amplitude signal using the updated header information.
According to a second aspect of the invention there is provided a frame structure for an optical packet for transmission over an optical network, comprising a packet header and a packet payload, wherein data is stored in the packet payload at a higher rate than in the packet header, and wherein the data in the header is encoded using amplitude modulation.
Preferably, low modulation depth amplitude modulation is employed, and the carrier wavelengths for the header and the payload are the same.